The present invention relates to a rare-earth element oxide phosphor suitable for use in a radiation detector for detecting X-rays, y rays and the like and particularly for use in the radiation detector of an X-ray CT apparatus, a positron camera or the like. The present invention also relates to a radiation detector and an X-ray CT apparatus using the phosphor.
As the radiation detectors used in X-ray CT apparatuses and the like there have conventionally been used ones combining a xenon gas chamber or BGO (bismuth germanium oxide) single crystal and a photomultiplier tube or combining CsI:Tl single crystal or CdWO4 single crystal and a photodiode. Properties generally required of a scintillator material used in a radiation detector include short afterglow, high luminous efficiency, high X-ray stopping power and chemical stability. The aforementioned single crystal phosphor, however, has variations in its characteristics and drawbacks in any of deliquescence, cleavage, afterglow (emission after X-ray irradiation is stopped) phenomenon, luminous efficiency and the like.
In recent years, however, rare-earth-system phosphors with high radiation-to-light conversion efficiencies have been developed as scintillators and radiation detectors combining such a phosphor with a photodiode have been put into practical use. Rare-earth phosphors consist of rare-earth element oxide or rare-earth element oxysulfide as base material and an activator as luminescence component. As a rare-earth element oxide phosphor, a phosphor including yttrium oxide or gadolinium oxide as base material has been proposed (Japanese Patent Publication No. 63(1988)-59436, Japanese Unexamined Patent Publication No.3 (1991)-50991, for example). As a rare-earth element oxysulfide phosphor, phosphors including Pr or Ce as the activator have been proposed (Japanese Patent Publication No. 60(1985)-4856).
Although these rare-earth-system phosphors include a phosphor having a good luminous efficiency, sufficient performance cannot always be achieved in such applications as X-ray CT which requires a phosphor having a very short afterglow time (a time required for light to attenuate to {fraction (1/10)} after X-ray irradiation is stopped).
For solving this problem, the inventors has developed and proposed phosphors including gadolinium (Gd), aluminum (Al) and gallium (Ga) as base material and cerium (Ce) as luminescence component, which has high X-ray stopping power and good wavelength matching with photodetectors (Japanese Patent Application No.9(1997)-355073, International Patent Application WO99/33934).
An object of the present invention is therefore to provide a phosphor with very short afterglow and high luminous efficiency that is particularly useful as a scintillator in X-ray CT and the like based on the above-mentioned GdAlGa:Ce phosphor. Another object of the present invention is to provide a radiation detector that is equipped with the phosphor and is high in detection efficiency. Another object of the present invention is to provide an X-ray CT apparatus that is equipped with a radiation detector with high luminous efficiency as a radiation detector and can provide high-resolution, high-quality tomographic images.
In order to achieve the foregoing objects, the inventors conducted an intense study aiming at improving properties such as luminous efficiency and afterglow of the GdAlGa:Ce phosphor including Ce as luminescence component and found as a result that luminous efficiency of the above-mentioned GdAlGa:Ce phosphor can be markedly improved by adding a certain amount of scandium (Sc) to the phosphor.
Specifically, the phosphor of the present invention is a phosphor represented by the general formula
(Gd1-x-yLxCeyScz)3Al5-dGadO12 
where L represents La or Y, and x, y, z and d are values falling in the ranges of 0xe2x89xa6x less than 0.2, 0.0005xe2x89xa6yxe2x89xa60.05, 0 less than zxe2x89xa60.03, and 0 less than d less than 5.
The phosphor of the present invention is a phosphor represented by the general formula
(Gd1-y-zCeyScz)3Al5-dGadO12 
where y, z and d are values falling in the ranges of 0.0005xe2x89xa6y less than 0.05, 0 less than zxe2x89xa60.03, and 0 less than d less than 5.
The phosphor of the present invention includes Ce as an activator (luminescence component) and absorbs radiation such as X-rays and gamma rays, exhibits yellowish emission due to Ce ion. When the phosphor is used as a scintillator of a radiation detector, matching with the photodiode is relatively good and a luminous output can be obtained that is 1.8 times or more than that of the CdWO4 currently widely used as a scintillator for X-ray CT. The luminous output is about 1.2 times that of the GdAlGa:Ce phosphor disclosed in Japanese Patent Application No.9(1997)-355073.
The phosphor is markedly low in afterglow since it contains Ce as luminous ion and its emission attenuates to 10% by about 220 ns (nano-seconds) after X-ray irradiation is stopped and to 2xc3x9710xe2x88x925 by about 30 ms. Generally phosphor afterglow includes primary afterglow and secondary afterglow (long-afterglow component. In X-ray CT, the secondary afterglow is problematic because information-carrying signals (X-ray) become indistinct in the time-axis direction. The phosphor is markedly low in the secondary afterglow (afterglow after 30 ms), i.e., 2xc3x9710xe2x88x925, and therefore excellent in properties suitable for scintillators of X-ray CT.
In the phosphor of the present invention, part of the element gadolinium (Gd) can be replaced with the element lanthanum (La) and/or the element yttrium (Y). In this case, the phosphor remains markedly low in afterglow. However, the content of La or Y (ratio z replacing Gd) should be less than 0.2, preferably less than 0.1, since as the content increases, the luminous efficiency and X-ray stopping power are degraded. The luminous efficiency and X-ray stopping power can be maximized when La or Y is not included.
Scandium (Sc) markedly improves luminous efficiency of the GdAlGa:Ce phosphor when it is included therein. Specifically, the luminous efficiency becomes about 20% higher than that of the phosphor including no Sc. The effect of Sc can be obtained by adding even a very small amount of Sc. However, if the content (z) of Sc exceeds 0.03, the luminous efficiency lowers to the same level as that of the phosphor including no Sc. Accordingly, the content (z) should be 0.03 or less.
By using aluminum (Al) together with gallium (Ga), high luminous efficiency can be obtained. According to the inventors"" investigation, it was found that when Gd-oxide-system phosphors containing Ce as luminous component include only one of Al and Ga, that is, base material is Gd3Al5O12 or Gd3Ga5O12, they do not exhibit practical amount of emission contrary to YAG-system. However, once Al and Ga were coexistent in the phosphor, the phosphor becomes to exhibit emission and, in addition, have markedly low afterglow. The total content of Al (5-d) and Ga (d) is 5 to (Gd+L+Ce+Sc)=3 in atomic ratio, and d satisfies 0 less than d less than 5, preferably 1.7 less than d less than 3.3, more preferably 2xe2x89xa6dxe2x89xa63. When the Al content and Ga content are within the range of from 1.7 to 3.3 respectively, a luminous output that is 1.8 times or more than that of the CdWO4 can be obtained.
Ce is an element that serves as an activator (luminescence component) in the phosphor of the present invention. The Ce content for generating Ce emission (y) is 0.0005 or greater, preferably 0.001 or greater. The Ce content (y) is defined as 0.05 or less for applications requiring high luminous output because a luminous output 2 times that of CdWO4 cannot be obtained when the Ce content (y) exceeds 0.05. Preferably, the Ce content (y) is defined as 0.02 or less, more preferably 0.015 or less.
The phosphor of the present invention may contain other elements inevitably included therein. For example, when Gd2O3 is used as a starting material for manufacturing the phosphor of the present invention, Gd2O3 having purity of 99.99% may include 5 wtppm or less of such impurities as Eu2O3, Tb4O7 and, therefore, the phosphor may include such impurities.
The phosphor of the present invention is not particularly limited with regard to crystal morphology. It may be single crystal or polycrystal. The polycrystal is preferred in view of easiness of producing and small variation in characteristics. The process for producing other phosphors as single crystal reported in J. Appl. Phys., vol.42, p3049 (1971) can be applied as the process for preparing the phosphor of the present invention as single crystal. The phosphor is obtained as a sintered material by hot-pressing (HP) process which adds an appropriate sintering agent to scintillator powder (starting material) and presses it under conditions of a temperature of 1,400-1,700xc2x0 C., and a pressure of about 300-1,400 atm, or by hot-isostatic pressing (HIP) process under the same condition as that of the HP. This enables the phosphor to be obtained as a dense sintered body of high optical transmittance. Since the phosphor of the present invention is cubic crystal and not anisotropic in refractive index, it becomes to have high optical transmittance when it is made into a sintered body.
The phosphor (scintillator powder) before sintering can be prepared as follows: mixing Gd2O3, Ce2(C2O4)3.9H2O, Sc2O3, Al2O3 and Ga2O3, for example, as starting material powder in a stoichiometric ratio, occasionally adding an appropriate flux component, and conducting baking in an alumina crucible at a temperature of from 1,500xc2x0 C. to 1,700xc2x0 C. for several hours. Using the scintillator powder baked in this way, the sintered body is prepared as aforementioned.
The flux component is added in order to lower the melting temperature of the starting materials and expedite crystallization. As the flux component, BaF2 used for sintering the YAG-system phosphor and potassium compounds such as potassium salts can be used alone or as a mixture. The potassium compounds such as K2SO4, KNO3, K2CO3, K3PO4 are preferable.
The phosphor produced in this manner is dense, high in optical transmittance, and small variations in its characteristics. A radiation detector of large luminous output can therefore be obtained. Although the phosphor of the present invention can be used in intensifying screens, fluorescent screens, scintillators and other general phosphor applications, it is particularly suitable for use in X-ray CT detectors, which require high luminous output and small afterglow.
The radiation detector of the present invention is equipped with a ceramic scintillator and a photodetector for detecting scintillator emission. The phosphor described in the foregoing is used as the ceramic scintillator. A photodiode such as a PIN photodiode is preferably used as the photodetector. These photodiodes have high sensitivity and short response. Moreover, as they have wavelength sensitivity from the visible light to near infrared region, they are suitable for their good wavelength matching with the phosphor of the present invention.
The X-ray CT apparatus of the present invention is equipped with an X-ray source, an X-ray detector disposed facing the X-ray source, a revolving unit for holding the X-ray source and the X-ray detector and revolving them about the object to be examined, and image reconstruction means for reconstructing a tomographic image of the object based on the intensity of the X-rays detected by the X-ray detector, which CT apparatus uses as the X-ray detector a radiation detector combining the aforesaid phosphor and a photodiode.
High-quality, high-resolution images can be obtained by utilizing this X-ray detector because the high X-ray detection rate makes it possible to achieve an approximate doubling of sensitivity compared with an X-ray CT apparatus using a conventional scintillator (such as CdWO4) and also because its afterglow is extremely small.